The invention relates to a hydraulic circuit for controlling at least one lower-load consumer and one higher-load consumer in accordance with the preamble of claim 1.
Such circuits (also termed load-sensing circuits) are i.a. used for controlling mobile machines, for example excavators. By means of the central circuit, hydraulically actuated units of the machine, for example a rotating mechanism, the travelling mechanism, a shovel, an arm or clamping means mounted on the excavator boom are controlled.
A load-sensing circuit of this type is, for example, known from EP 0 566 449 AS. This circuit includes a variable displacement pump which may be controlled such as to generate at its output a pressure which exceeds the highest load pressure of the hydraulic consumer by a specific differential amount. For the purpose of regulation a load-sensing regulator is provided which may receive application of the pump pressure in the direction of reducing the stroke volume, and the highest pressure at the consumers, as well as a pressure spring in the direction of increasing the stroke volume. The difference between the pump pressure and the highest load pressure which occurs in the variable displacement pump corresponds to the force of the aforementioned pressure spring.
To each one of the consumers an adjustable metering orifice including a pressure compensator arranged downstream thereof is associated, whereby the pressure drop at the metering orifice is maintained constant, so that the amount of hydraulic fluid flowing to the respective consumer depends not on the load pressure of the consumer or the pump pressure but on the cross-section of opening of the metering orifice. In the case in which the variable displacement pump conveys at maximum volume while the hydraulic fluid flow nevertheless is not sufficient for maintaining the predetermined pressure drop across the metering orifices, the pressure compensators of all actuated hydraulic consumers are adjusted in a closing direction, so that any flow of hydraulic fluid to the individual consumers is reduced by an identical proportion. Namely, in the case of a downstream pressure compensator, the volume flows towards the consumers will always be proportional with the cross-section of opening of the metering orifices. Owing to this load-independent throughput distribution (LUDV), all controlled consumers move with a velocity reduced by an identical percentage.
The variable displacement pump mentioned at the outset is customarily equipped with a pressure control and with a power control whereby the maximum possible pump pressure or the maximum power capable of being output by the variable displacement pump (excavator power), respectively, may be adjusted. These pressure and power controls are superseded to the load-sensing regulation.
In the case of a control arrangement of the above described type, problems may occur when a hydraulic consumer works against a practically infinite resistance. This may, for example, be the case if the hydraulic consumer is a shovel being actuated against a stop. In the case of actuation against a stop, a pressure about corresponding to the maximum pressure (excavator power) predetermined by the pressure control builds up at the corresponding hydraulic consumer. If, now, an additional hydraulic consumer, for example a travelling mechanism or a boom is activated, the latter may only be displaced with a lower velocity, for owing to the high pressure at the former consumer (shovel), the power control of the variable displacement pump already responds at low flows of hydraulic fluid to the other hydraulic consumer (travelling mechanism).
In order to eliminate this drawback, a control arrangement is disclosed in WO95/32364 to the same applicant, by means of which only the load pressure of the lower-load hydraulic consumer is reported to the load-sensing regulator of the variable displacement pump when a limit load pressure is exceeded. This limit load pressure is selected such that the supply for the additional hydraulic consumer is ensured. In the subject matter of WO95/32364 this is achieved in that the spring cavity of the pressure compensator of the lower-load consumer may be connected to the reservoir via a pressure control valve arrangement. When a limit load pressure is exceeded, the pressure control valve opens the connection to the reservoir, so that the spring cavity of the pressure compensator of the lower-load consumer is relieved of pressure, and the control piston is taken into its open position wherein the load pressure of this consumer is reported in the load pressure reporting line.
It is a drawback in this control arrangement that a partial volume flow is discharged towards the reservoir and thus is not available for consumer control. The efficiency of this control is accordingly comparatively low. It is another drawback that owing to hydraulic fluid being returned towards the reservoir, heat is generated in the system and thus pump power is dissipated.
In contrast, the invention is based on the object of furnishing a control arrangement whereby sufficient supply of all consumers is ensured at minimum expense in terms of device technology.
This object is attained through a hydraulic circuit having the features of claim 1.
Owing to the measure of providing a bypass channel through which the pressure compensator downstream from the metering orifice may be bypassed, it is not necessary to establish a lower setting of the pressure compensator, or discharge hydraulic fluid into the reservoir in order to limit the system pressure. The manifesting system pressure may be predetermined by corresponding selection of the bypass cross-section. On account of the reduced system pressure, the lower-load consumer may be supplied with a greater amount of hydraulic fluid which may be utilized, for example, for increasing a velocity of a boom or the like.
A circuit having a particularly simple construction is obtained if the metering orifice upstream from the pressure compensator is formed by a proportional directional control valve, with the bypass channel being capable of being controlled open in accordance with the valve spool position of the proportional directional control valve. Due to the fact that the bypass channel is controlled open in dependence on control of the proportional valve, the individual-pressure compensator acts merely in the fine control range where comparatively low hydraulic fluid volume flows pass through the pressure compensator.
The construction may be simplified further if the bypass channel is formed in the valve spool of the proportional directional control valve and may be controlled open by a control land of the valve spool bore.
In order to prevent return flow from the consumer through the bypass channel, a check valve arrangement is provided in the latter.
In a preferred variant of the invention, two work ports of a consumer are controlled through the proportional valve. In some cases, e.g., in the case of double-action hydraulic cylinders, it is sufficient if the bypass channel is associated with only one of the work ports, so that a flow through the bypass takes place, for example in the lifting function. It is, of course, also possible to associate bypass channels to both work ports.
As was already mentioned above, it may be advantageous if the bypass channel is controlled open only following a specific stroke of the proportional valve, so that no bypass flow is engendered at the beginning of the control.
The valve spool of the proportional directional control valve is preferably designed to include a central velocity component and two external directional components each associated with one port of the consumer. The bypass channel in this case extends inside the valve spool from the velocity component towards the directional component, so that the pressure compensator is bypassed.
The pressure loss in the bypass channel may be minimized if the latter has oblique and radial bores opening into the outer periphery of the valve spool.
Other advantageous developments of the invention are subject matters of the further appended claims.